Using procedures to clone fragments of sea urchin DNA in bacterial plasmids, sequences expressed as mRNA molecules whose appearance is regulated during embryogenesis will be selected. Total sea urchin DNA will be cleaved with restriction endonuclease and inserted into plasmids. Bacteria carrying appropriate sea urchin DNA sequences will be selected by hybridizing radioactive mRNA from different stages of sea urchin development to colonies on replicate filters to identify clones which react to mRNA from one stage of development but not at another.